Effects of Crystallinity on the Photocatalytic Polymerization of 3,4-Ethylenedioxythiophene over CsPbBr3 Inverse Opals

: Due to their high absorption coefﬁcient and long carrier lifetime, halide perovskites are promising candidates for photocatalysts. For this study, the antisolvent crystallization protocol and the colloidal crystal templating approach were combined to fabricate the highly crystalline cesium lead bromide perovskite with inverse opal morphology (IO-CsPbBr 3 ). Scanning electron microscopy and transmission electron microscope images demonstrate the three-dimensional well-ordered porous structures of the IO-CsPbBr 3 and their single-crystalline features. The presented approach not only provides hierarchical porous structures but also enhances overall crystallinity. When used as catalysts to promote the polymerization of 2,2 (cid:48) ,5 (cid:48) ,2”-ter-3,4-ethylenedioxythiophene, the highly crystalline IO-CsPbBr 3 exhibits a superior photocatalytic performance compared to its polycrystalline counterpart. Furthermore, the morphology and the crystalline structure of the highly crystalline IO-CsPbBr 3 are well preserved under photocatalytic conditions. This novel approach enables the preparation of a halide perovskite inverse opal with high crystallinity.


Introduction
In the past decade, halide perovskites have been regarded as a fascinating optoelectronic material, and their excellent properties have reformed the development of the photovoltaic and light-emitting diodes field [1][2][3]. Through their high absorbance coefficient [4] and long diffusion lifetime [5], halide perovskites also show their potential as photocatalysts. Efforts have been devoted to exploring their application in photocatalytic CO 2 reduction [6][7][8], α-alkylation of aldehydes [9], the activation of C(sp 3 )-H [10], the degradation of organic dyes [11], the polymerization of 2,2 ,5 ,2"-ter-3,4-ethylenedioxythiophene (TerEDOT) [12] and the oxidation of benzyl alcohol [13,14]. Most of the examples have focused on the photocatalytic performance of halide perovskite quantum dots, and the influence of the surface state and halide composition on their photocatalytic performance has been reported [15,16]. Halide perovskite quantum dots possess a single-crystalline nature and a high surface area, which benefits photocatalytic performance. However, due to its high surface energy, the nanoscale halide perovskite may agglomerate. To preserve the morphology of the quantum dots, a surfactant is necessary, which unfortunately may make the electron transfer across the interface sluggish. To this end, a highly crystalline halide perovskite on a micrometer, or even on a larger scale, may circumvent this problem. However, halide perovskites on a large scale have been rarely reported.
The morphology and crystallinity of photocatalysts are two important factors relating to their performance [17,18]. It is well known that hierarchical structure is desirable for the photocatalyst since it may provide more active sites and shorten the carrier diffusion length between the carrier-generated center and the active sites [19]. The inverse opal has been proven to be an advantageous morphology for photocatalysts [20][21][22][23]. Tüysüz et al. developed the colloidal templating method to prepare halide perovskite inverse opals [24]. The photocatalytic degradation of organic dye over the CsPbBr 3 inverse opal has been investigated, and the enhanced performance of the hierarchical structure has been demonstrated [25]. Meanwhile, crystallinity plays an important role in the performance of optoelectronic materials [26]. Single crystals of halide perovskite also exhibited low trap density and long diffusion length and, thus, are desirable for preparing high-performance materials and devices [27]. However, the preparation of single crystalline optoelectronic materials with hierarchical structures is highly challenging. Snaith's group has reported single crystalline mesoporous TiO 2 for enhanced mobility and device performance [28]. To date, neither the fabrication of hierarchical single-crystalline all-inorganic halide perovskites nor the effect of the crystallinity of halide perovskites on their photocatalytic performance has been reported.
In this work, four colloidal crystals were prepared by using polystyrene microspheres with different diameters, and their long-ranged, ordered arrangement was evidenced by distinct photonic stop bands. Using the colloidal crystals as templates, a series of highly crystalline and polycrystalline CsPbBr 3 inverse opals were prepared. The obtained perovskite inverse opals were employed as photocatalysts to accelerate the polymerization of TerEDOT, and the highly crystalline CsPbBr 3 inverse opals exhibited enhanced photocatalytic performance compared to their polycrystalline counterparts. The significant improvement could be attributed to a higher degree of crystallinity and the inverse opal morphology.

Results and Discussion
Highly crystalline cesium lead bromide (IO-CsPbBr 3) with inverse opal structure was synthesized by combining the anti-solvent method and the colloidal crystal templating approach (as shown in Scheme 1: synthesis details are presented in the Experimental Section). A series of artificial opal templates were fabricated by filtrating the polystyrene microsphere with different diameters (214 nm, 379 nm, 433 nm and 501 nm) [29]. The distinct photonic stop bands demonstrated the ordered arrangement of the artificial templates. Later, the templates were immersed into the DMSO solution, which contained equimolar amounts of CsBr and PbBr 2 , and further treated with methanol. After 24 h at 40 • C, the light-orange-colored composites were obtained, which indicated the formation of CsPbBr 3 . Finally, the polystyrene was removed with toluene, and the powder was collected by centrifugation. The samples were prepared by anti-solvent and were denoted as AS-IO-CsPbBr 3 . Reference samples were prepared by directly removing the DMSO under vacuum (see Experimental Section for details), which were denoted as RS-IO-CsPbBr 3 . Moreover, the AS-IO-CsPbBr 3 and RS-IO-CsPbBr 3 were synthesized by using the artificial opal template of PSs with diameters of 214 nm, 379 nm, 433 nm and 501 nm, which were denoted as 2/IO-CsPbBr 3 , 3/IO-CsPbBr 3 , 4/IO-CsPbBr 3 and 5/IO-CsPbBr 3 , respectively. al. developed the colloidal templating method to prepare halide perovskite inverse opals [24]. The photocatalytic degradation of organic dye over the CsPbBr3 inverse opal has been investigated, and the enhanced performance of the hierarchical structure has been demonstrated [25]. Meanwhile, crystallinity plays an important role in the performance of optoelectronic materials [26]. Single crystals of halide perovskite also exhibited low trap density and long diffusion length and, thus, are desirable for preparing high-performance materials and devices [27]. However, the preparation of single crystalline optoelectronic materials with hierarchical structures is highly challenging. Snaith's group has reported single crystalline mesoporous TiO2 for enhanced mobility and device performance [28].
To date, neither the fabrication of hierarchical single-crystalline all-inorganic halide perovskites nor the effect of the crystallinity of halide perovskites on their photocatalytic performance has been reported. In this work, four colloidal crystals were prepared by using polystyrene microspheres with different diameters, and their long-ranged, ordered arrangement was evidenced by distinct photonic stop bands. Using the colloidal crystals as templates, a series of highly crystalline and polycrystalline CsPbBr3 inverse opals were prepared. The obtained perovskite inverse opals were employed as photocatalysts to accelerate the polymerization of TerEDOT, and the highly crystalline CsPbBr3 inverse opals exhibited enhanced photocatalytic performance compared to their polycrystalline counterparts. The significant improvement could be attributed to a higher degree of crystallinity and the inverse opal morphology.

Results and Discussion
Highly crystalline cesium lead bromide (IO-CsPbBr3) with inverse opal structure was synthesized by combining the anti-solvent method and the colloidal crystal templating approach (as shown in Scheme 1: synthesis details are presented in the Experimental Section). A series of artificial opal templates were fabricated by filtrating the polystyrene microsphere with different diameters (214 nm, 379 nm, 433 nm and 501 nm) [29]. The distinct photonic stop bands demonstrated the ordered arrangement of the artificial templates. Later, the templates were immersed into the DMSO solution, which contained equimolar amounts of CsBr and PbBr2, and further treated with methanol. After 24 h at 40 °C, the light-orange-colored composites were obtained, which indicated the formation of CsP-bBr3. Finally, the polystyrene was removed with toluene, and the powder was collected by centrifugation. The samples were prepared by anti-solvent and were denoted as AS-IO-CsPbBr3. Reference samples were prepared by directly removing the DMSO under vacuum (see Experimental Section for details), which were denoted as RS-IO-CsPbBr3. Moreover, the AS-IO-CsPbBr3 and RS-IO-CsPbBr3 were synthesized by using the artificial opal template of PSs with diameters of 214 nm, 379 nm, 433 nm and 501 nm, which were denoted as 2/IO-CsPbBr3, 3/IO-CsPbBr3, 4/IO-CsPbBr3 and 5/IO-CsPbBr3, respectively. Scheme 1. Preparation route of IO-CsPbBr3.
The highly ordered colloidal crystal template is a prerequisite for preparing the halide perovskites with inverse opal morphology. An ordered, close-packed structure can be The highly ordered colloidal crystal template is a prerequisite for preparing the halide perovskites with inverse opal morphology. An ordered, close-packed structure can be evidenced by distinct stop bands, whose wavelength is governed by Bragg's law and depends on lattice spacing, mainly on the diameter of the microspheres [30]. The scanning electronic microscopy (SEM) image exhibited the highly ordered (111) faces of  Figure 1A) and the long-ranged arrangement of the close-packed templates (Supplementary Materials, Figure S1). The reflectance spectra of the samples with different diameters are shown in Figure 1B. As the diameters of microspheres increased from 379 nm to 501 nm, the stop band position shifted from 473 nm to 668 nm. This behavior can be expressed by Bragg's law: mλ = 2ndsinθ (1) where m is the order of diffraction, λ is the wavelength of the diffracted light, n is the refractive index of the crystalline colloidal arrays, d is the layer spacing, and θ is the angle between the incident light and the diffracting lattice plane. The stop band wavelength of the template with the microsphere diameter of 214 nm was 550 nm, which could be ascribed to the different order of diffraction compared with the other three opal structured templates [31]. As shown in Figure S1, low-resolution scanning electronic microscopy (SEM) images revealed that the template displayed a 3D ordered structure. In the infiltration process, DMSO was selected owing to its incompatibility with polystyrene but good compatibility with halide perovskites at low temperatures. Meanwhile, methanol was used as anti-solvent in the crystallization process because the polystyrene microspheres CsBr and PbBr 2 show poor solubility in methanol. After the template was removal by toluene, a perovskite inverse opal replicate with an ordered structure was obtained (Figure 2A,B).
electronic microscopy (SEM) image exhibited the highly ordered (111) faces of plate ( Figure 1A) and the long-ranged arrangement of the close-packed templa plementary Materials, Figure S1). The reflectance spectra of the samples with diameters are shown in Figure 1B. As the diameters of microspheres increased nm to 501 nm, the stop band position shifted from 473 nm to 668 nm. This beha be expressed by Bragg's law: where m is the order of diffraction, λ is the wavelength of the diffracted light refractive index of the crystalline colloidal arrays, d is the layer spacing, and θ is t between the incident light and the diffracting lattice plane. The stop band wave the template with the microsphere diameter of 214 nm was 550 nm, which cou cribed to the different order of diffraction compared with the other three opal st templates [31]. As shown in Figure S1, low-resolution scanning electronic mi (SEM) images revealed that the template displayed a 3D ordered structure. In the tion process, DMSO was selected owing to its incompatibility with polystyrene b compatibility with halide perovskites at low temperatures. Meanwhile, metha used as anti-solvent in the crystallization process because the polystyrene micr CsBr and PbBr2 show poor solubility in methanol. After the template was remov uene, a perovskite inverse opal replicate with an ordered structure was obtained 2A,B).  The SEM images in Figure 2A clearly show that the 2/AS-IO-CsPbBr3 samp entirely perforated with the well-ordered inverse opal structure. The inner vo topologically interconnected, and the well-defined 3D ordered macropores were evidenced by distinct stop bands, whose wavelength is governed by Bragg's law and depends on lattice spacing, mainly on the diameter of the microspheres [30]. The scanning electronic microscopy (SEM) image exhibited the highly ordered (111) faces of the template ( Figure 1A) and the long-ranged arrangement of the close-packed templates (Supplementary Materials, Figure S1). The reflectance spectra of the samples with different diameters are shown in Figure 1B. As the diameters of microspheres increased from 379 nm to 501 nm, the stop band position shifted from 473 nm to 668 nm. This behavior can be expressed by Bragg's law: where m is the order of diffraction, λ is the wavelength of the diffracted light, n is the refractive index of the crystalline colloidal arrays, d is the layer spacing, and θ is the angle between the incident light and the diffracting lattice plane. The stop band wavelength of the template with the microsphere diameter of 214 nm was 550 nm, which could be ascribed to the different order of diffraction compared with the other three opal structured templates [31]. As shown in Figure S1, low-resolution scanning electronic microscopy (SEM) images revealed that the template displayed a 3D ordered structure. In the infiltration process, DMSO was selected owing to its incompatibility with polystyrene but good compatibility with halide perovskites at low temperatures. Meanwhile, methanol was used as anti-solvent in the crystallization process because the polystyrene microspheres CsBr and PbBr2 show poor solubility in methanol. After the template was removal by toluene, a perovskite inverse opal replicate with an ordered structure was obtained ( Figure  2A,B).  The SEM images in Figure 2A clearly show that the 2/AS-IO-CsPbBr3 samples were entirely perforated with the well-ordered inverse opal structure. The inner voids were topologically interconnected, and the well-defined 3D ordered macropores were approx- The SEM images in Figure 2A clearly show that the 2/AS-IO-CsPbBr 3 samples were entirely perforated with the well-ordered inverse opal structure. The inner voids were topologically interconnected, and the well-defined 3D ordered macropores were approximately 200 nm, which corresponded to the diameter of the original colloidal microspheres ( Figure 2B). Compared to 2/RS-IO-CsPbBr 3 ( Figure 2C), 2/AS-IO-CsPbBr 3 showed a more regular inverse opal morphology along with a narrower wall thickness and less non-structured solid material. The smaller quantity of non-structured solid material is beneficial for photocatalytic activities. On the one hand, the non-structured solids reduced the ability of the reactant to approach the surface of the halide perovskites. On the other hand, the photogenerated carriers in the non-structured solid material have to diffuse a long way through the material before they reach the surface, which increases the possibility of their recombination [25]. Similar phenomena were also observed from the SEM images of other 3/IO-CsPbBr 3 , 4/IO-CsPbBr 3 and 5/IO-CsPbBr 3 samples (Supplementary Materials, Figure S1). In the process of infiltration, three different concentrations of perovskite precursor (0.30, 0.45 and 0.6 M) in DMSO solution were studied to optimize the crystallization process. When the concentration of precursor was 0.3 M, porous halide perovskites were observed, but the fraction of inverse opal was low, and the macropores of the material did not connect with each other. By contrast, at higher concentrations of 0.6 M, the fraction of non-structured solid material was increased. The inverse opal structure was prepared by the precursor solution containing 0.45 M PbBr 2 , and CsBr (Supplementary Materials, Figure S2) showed the best morphology.
In order to investigate the phase of the CsPbBr 3 crystal, the PXRD patterns of the synthesized crystalline powders are shown in Figure 3 [32,33]. The XRD patterns of AS-IO-CsPbBr 3 samples ( Figure 3A) matched well with the standard reflection of CsPbBr 3 . Moreover, the clear splitting of the (110) and (220) reflections indicated that the grown CsPbBr 3 crystal was in the room-temperature orthorhombic phase [34]. Meanwhile, the XRD patterns of RS-IO-CsPbBr 3 samples ( Figure 3B) show similar reflections to those of highly crystalline IO-CsPbBr 3 , which indicates that the RS-IO-CsPbBr 3 samples are also in the orthorhombic phase. Although the PXRD patterns confirmed that all the IO-CsPbBr 3 samples are orthorhombic, the difference between the AS-IO-CsPbBr 3 and RS-IO-CsPbBr 3 samples is indistinctive. imately 200 nm, which corresponded to the diameter of the original colloidal microspheres ( Figure 2B). Compared to 2/RS-IO-CsPbBr3 ( Figure 2C), 2/AS-IO-CsPbBr3 showed a more regular inverse opal morphology along with a narrower wall thickness and less non-structured solid material. The smaller quantity of non-structured solid material is beneficial for photocatalytic activities. On the one hand, the non-structured solids reduced the ability of the reactant to approach the surface of the halide perovskites. On the other hand, the photogenerated carriers in the non-structured solid material have to diffuse a long way through the material before they reach the surface, which increases the possibility of their recombination [25]. Similar phenomena were also observed from the SEM images of other 3/IO-CsPbBr3, 4/IO-CsPbBr3 and 5/IO-CsPbBr3 samples (Supplementary Materials, Figure S1). In the process of infiltration, three different concentrations of perovskite precursor (0.30, 0.45 and 0.6 M) in DMSO solution were studied to optimize the crystallization process. When the concentration of precursor was 0.3 M, porous halide perovskites were observed, but the fraction of inverse opal was low, and the macropores of the material did not connect with each other. By contrast, at higher concentrations of 0.6 M, the fraction of non-structured solid material was increased. The inverse opal structure was prepared by the precursor solution containing 0.45 M PbBr2, and CsBr (Supplementary Materials, Figure S2) showed the best morphology. In order to investigate the phase of the CsPbBr3 crystal, the PXRD patterns of the synthesized crystalline powders are shown in Figure 3 [32,33]. The XRD patterns of AS-IO-CsPbBr3 samples ( Figure 3A) matched well with the standard reflection of CsPbBr3. Moreover, the clear splitting of the (110) and (220) reflections indicated that the grown CsPbBr3 crystal was in the room-temperature orthorhombic phase [34]. Meanwhile, the XRD patterns of RS-IO-CsPbBr3 samples ( Figure 3B) show similar reflections to those of highly crystalline IO-CsPbBr3, which indicates that the RS-IO-CsPbBr3 samples are also in the orthorhombic phase. Although the PXRD patterns confirmed that all the IO-CsPbBr3 samples are orthorhombic, the difference between the AS-IO-CsPbBr3 and RS-IO-CsPbBr3 samples is indistinctive. To further investigate the crystallinity of both AS-IO-CsPbBr3 and RS-IO-CsPbBr3, TEM measurements were conducted. As shown in Figure 4A, the well-defined structure of 2/AS-IO-CsPbBr3 could be observed at low magnification, and this result is also consistent with SEM images. the hexagonally aligned macropores inherited the (111) planes of the colloidal crystal template can be clearly identified in the enlarged image ( Figure  4B), thus indicating that the top surface of the inverse opal crystal is perpendicular to the electron beam. The HR-TEM images show the interplanar distances of 2/AS-IO-CsPbBr3 were 4.1 Å , corresponding to the (112) face of CsPbBr3. Sharp diffraction spots in the corresponding SAED pattern ( Figure 4C) can be indexed to the orthorhombic perovskite crystal structure [35]. Indeed, the electron diffraction data obtained from other regions of the sample showed exactly the same patterns, which demonstrates that the complex structure To further investigate the crystallinity of both AS-IO-CsPbBr 3 and RS-IO-CsPbBr 3 , TEM measurements were conducted. As shown in Figure 4A, the well-defined structure of 2/AS-IO-CsPbBr 3 could be observed at low magnification, and this result is also consistent with SEM images. the hexagonally aligned macropores inherited the (111) planes of the colloidal crystal template can be clearly identified in the enlarged image ( Figure 4B), thus indicating that the top surface of the inverse opal crystal is perpendicular to the electron beam. The HR-TEM images show the interplanar distances of 2/AS-IO-CsPbBr 3 were 4.1 Å, corresponding to the (112) face of CsPbBr 3 . Sharp diffraction spots in the corresponding SAED pattern ( Figure 4C) can be indexed to the orthorhombic perovskite crystal structure [35]. Indeed, the electron diffraction data obtained from other regions of the sample showed exactly the same patterns, which demonstrates that the complex structure actually has a single crystalline nature. The TEM images and the SEAD patterns of the 3/AS-IO-CsPbBr 3 , 4/AS-IO-CsPbBr 3 and 5/AS-IO-CsPbBr 3 samples also exhibited well-defined structures and sharp spots (Supplementary Materials, Figure S3). In contrast, the TEM images of the RS-IO-CsPbBr 3 samples were less ordered. More importantly,  Figure S4) were also sharp, but the arrangements were less regular, which strongly suggested the polycrystallinity of the CsPbBr 3 in the RS-IO-CsPbBr 3 samples. TEM measurements confirmed the higher crystallinity of AS-IO-CsPbBr 3 as compared to RS-IO-CsPbBr 3 samples. actually has a single crystalline nature. The TEM images and the SEAD patterns of the 3/AS-IO-CsPbBr3, 4/AS-IO-CsPbBr3 and 5/AS-IO-CsPbBr3 samples also exhibited well-defined structures and sharp spots (Supplementary Materials, Figure S3). In contrast, the TEM images of the RS-IO-CsPbBr3 samples were less ordered. More importantly, the diffraction spots on the SEAD image of RS-IO-CsPbBr3 ( Figure S4) were also sharp, but the arrangements were less regular, which strongly suggested the polycrystallinity of the CsPbBr3 in the RS-IO-CsPbBr3 samples. TEM measurements confirmed the higher crystallinity of AS-IO-CsPbBr3 as compared to RS-IO-CsPbBr3 samples. Since visible light (400 nm <λ <800 nm) takes up almost half of the solar energy incident on the earth's surface, absorption of photons within the visible light region is a desirable feature for photocatalysts. As shown in Figure 5A Figure 5B), respectively, which is consistent with the previous reports [36]. Meanwhile, as the diameter of the PS used to assemble the colloidal template increased (to 379 nm, 433 nm and 501 nm), the corresponding IO-CsPbBr3 samples displayed similar maximum absorbance wavelengths at approximately 560 nm (Supplementary Materials, Figure S5), which demonstrates that this inverse opal crystallization control method is compatible with polystyrene microspheres with different diameters and, thus, is feasible for controlling the morphology of inverse opal structure. Since visible light (400 nm <λ <800 nm) takes up almost half of the solar energy incident on the earth's surface, absorption of photons within the visible light region is a desirable feature for photocatalysts. As shown in Figure 5A Figure 5B), respectively, which is consistent with the previous reports [36]. Meanwhile, as the diameter of the PS used to assemble the colloidal template increased (to 379 nm, 433 nm and 501 nm), the corresponding IO-CsPbBr 3 samples displayed similar maximum absorbance wavelengths at approximately 560 nm (Supplementary Materials, Figure S5), which demonstrates that this inverse opal crystallization control method is compatible with polystyrene microspheres with different diameters and, thus, is feasible for controlling the morphology of inverse opal structure.
actually has a single crystalline nature. The TEM images and the SEAD patterns of the 3/AS-IO-CsPbBr3, 4/AS-IO-CsPbBr3 and 5/AS-IO-CsPbBr3 samples also exhibited well-defined structures and sharp spots (Supplementary Materials, Figure S3). In contrast, the TEM images of the RS-IO-CsPbBr3 samples were less ordered. More importantly, the diffraction spots on the SEAD image of RS-IO-CsPbBr3 ( Figure S4) were also sharp, but the arrangements were less regular, which strongly suggested the polycrystallinity of the CsPbBr3 in the RS-IO-CsPbBr3 samples. TEM measurements confirmed the higher crystallinity of AS-IO-CsPbBr3 as compared to RS-IO-CsPbBr3 samples. Since visible light (400 nm <λ <800 nm) takes up almost half of the solar energy incident on the earth's surface, absorption of photons within the visible light region is a desirable feature for photocatalysts. As shown in Figure 5A Figure 5B), respectively, which is consistent with the previous reports [36]. Meanwhile, as the diameter of the PS used to assemble the colloidal template increased (to 379 nm, 433 nm and 501 nm), the corresponding IO-CsPbBr3 samples displayed similar maximum absorbance wavelengths at approximately 560 nm (Supplementary Materials, Figure S5), which demonstrates that this inverse opal crystallization control method is compatible with polystyrene microspheres with different diameters and, thus, is feasible for controlling the morphology of inverse opal structure. To investigate the effect of crystallinity on the photocatalytic performance of halide perovskites, the polymerization of TerEDOT was chosen due to its potential application in optoelectronic device [12]. Similar to the photocatalytic processes of CsPbI 3 quantum dots, TerEDOT plays the role of the hole acceptor, while 1, 4-benzoquinone (Qu) was used as the electron acceptor. Under visible light illumination, the IO-CsPbBr 3 powders generated a hole and an electron on the valence band and conduction band, respectively. As the hole transferred to TerEDOT and the electron could be transferred to Qu, polymerization occurred (Scheme 2) [12]. In contrast to previous studies, the concentration of TerEDOT in the supernate was monitored by UV/Vis spectrum to analyze the reaction process. To investigate the effect of crystallinity on the photocatalytic performance of halide perovskites, the polymerization of TerEDOT was chosen due to its potential application in optoelectronic device [12]. Similar to the photocatalytic processes of CsPbI3 quantum dots, TerEDOT plays the role of the hole acceptor, while 1, 4-benzoquinone (Qu) was used as the electron acceptor. Under visible light illumination, the IO-CsPbBr3 powders generated a hole and an electron on the valence band and conduction band, respectively. As the hole transferred to TerEDOT and the electron could be transferred to Qu, polymerization occurred (Scheme 2) [12]. In contrast to previous studies, the concentration of TerEDOT in the supernate was monitored by UV/Vis spectrum to analyze the reaction process.   Figure S6 (see Experimental Section for details). During the photocatalytic reaction, the time-dependent spectra showed that the absorbance between 440 nm and 520 nm decreased, and the absorbance at 452 nm, indicating the concentration of the TerEDOT, could be used to monitor the reaction. The presented profiles reveal that highly crystalline 2/AS-IO-CsPbBr3 promoted the polymerization of TerEDOT more significantly than 2/RS-IO-CsPbBr3 ( Figure 6A,B). In the case of 2/AS-IO-CsPbBr3, the absorbance of the TerEDOT decreased by 34%, 45% and 55% within the first 15 min, 30 min and 45 min, respectively ( Figure 6C). In contrast, the polymerization catalyzed over 2/RS-IO-CsPbBr3 was slower. The absorbance of the TerEDOT decreased to only approximately 63% at 45 min, which is comparable to the polymerization catalyzed by highly crystalline 2/IO-CsPbBr3 within the first 15 min.   Figure S6 (see Experimental Section for details). During the photocatalytic reaction, the time-dependent spectra showed that the absorbance between 440 nm and 520 nm decreased, and the absorbance at 452 nm, indicating the concentration of the TerEDOT, could be used to monitor the reaction. The presented profiles reveal that highly crystalline 2/AS-IO-CsPbBr 3 promoted the polymerization of TerEDOT more significantly than 2/RS-IO-CsPbBr 3 ( Figure 6A,B). In the case of 2/AS-IO-CsPbBr 3 , the absorbance of the TerEDOT decreased by 34%, 45% and 55% within the first 15 min, 30 min and 45 min, respectively ( Figure 6C). In contrast, the polymerization catalyzed over 2/RS-IO-CsPbBr 3 was slower. The absorbance of the TerEDOT decreased to only approximately 63% at 45 min, which is comparable to the polymerization catalyzed by highly crystalline 2/IO-CsPbBr 3 within the first 15 min.

2/RS-IO-CsPbBr3.
To investigate the effect of crystallinity on the photocatalytic performance of halide perovskites, the polymerization of TerEDOT was chosen due to its potential application in optoelectronic device [12]. Similar to the photocatalytic processes of CsPbI3 quantum dots, TerEDOT plays the role of the hole acceptor, while 1, 4-benzoquinone (Qu) was used as the electron acceptor. Under visible light illumination, the IO-CsPbBr3 powders generated a hole and an electron on the valence band and conduction band, respectively. As the hole transferred to TerEDOT and the electron could be transferred to Qu, polymerization occurred (Scheme 2) [12]. In contrast to previous studies, the concentration of TerEDOT in the supernate was monitored by UV/Vis spectrum to analyze the reaction process.

Scheme 2.
Illustration of the proposed mechanism for photocatalytic polymerization of TerEDOT over IO-CsPbBr3 samples under visible light illumination. Figure 6A,B show the time-dependent absorbance spectra of the polymerization of TerEDOT over 2/AS-IO-CsPbBr3 and 2/RS-IO-CsPbBr3, respectively. The UV/Vis spectra of the photocatalytic reaction over 3/AS-IO-CsPbBr3, 4/AS-IO-CsPbBr3 and 5/AS-IO-CsP-bBr3 and the corresponding reference samples are shown in Figure S6 (see Experimental Section for details). During the photocatalytic reaction, the time-dependent spectra showed that the absorbance between 440 nm and 520 nm decreased, and the absorbance at 452 nm, indicating the concentration of the TerEDOT, could be used to monitor the reaction. The presented profiles reveal that highly crystalline 2/AS-IO-CsPbBr3 promoted the polymerization of TerEDOT more significantly than 2/RS-IO-CsPbBr3 ( Figure 6A,B). In the case of 2/AS-IO-CsPbBr3, the absorbance of the TerEDOT decreased by 34%, 45% and 55% within the first 15 min, 30 min and 45 min, respectively ( Figure 6C). In contrast, the polymerization catalyzed over 2/RS-IO-CsPbBr3 was slower. The absorbance of the TerEDOT decreased to only approximately 63% at 45 min, which is comparable to the polymerization catalyzed by highly crystalline 2/IO-CsPbBr3 within the first 15 min. These observations were attributed to the higher crystalline quality of 2/AS-IO-CsPbBr 3 , which contained lower defect density and fewer grain boundaries, and thus eliminated the recombination centers of the photogenerated electrons and holes [37]. Moreover, under the condition of adding the same weight of photocatalysts, 2/AS-IO-CsPbBr 3 had fewer non-structured solids for its more regular inverse opal structures (Figure 2), Catalysts 2021, 11, 1331 7 of 11 which consequently increased the number of active sites [38]. Additionally, the polymerization of TerEDOT under illumination in absence of IO-CsPbBr 3 powders is shown in Figure 6C, which reveals that TerEDOT undergoes extremely slow photolytic polymerization in the absence of a catalytic, causing a decrease in the TerEDOT concentration of approximately 4% within 45 min.
To further verify this phenomenon, photocatalytic polymerization catalyzed over other IO-CsPbBr 3 (3/IO-CsPbBr 3 , 4/IO-CsPbBr 3 and 5/IO-CsPbBr 3 ) samples was studied. As shown in Figure 7A,B, all of the AS-IO-CsPbBr 3 samples exhibit obviously higher photocatalytic performance than that of the corresponding RS-IO-CsPbBr 3 , which is consistent with the performance of 2/IO-CsPbBr 3 . Moreover, the photocatalytic performances of 3/RS-IO-CsPbBr 3 , 4/RS-IO-CsPbBr 3 and 5/RS-IO-CsPbBr 3 did not exhibit significant differences and were lower than that of 2/IO-CsPbBr 3 . The effect of morphology on catalytic performance was more significant in the case of AS-IO-CsPbBr 3 . In the presence of 3/AS-IO-CsPbBr 3 , 4/AS-IO-CsPbBr 3 and 5/AS-IO-CsPbBr 3 , the absorbance at 452 nm decreased to 64%, 56% and 50% at 45 min, respectively. This observation applies, in particular, to the area of AS-IO-CsPbBr 3 prepared using microspheres with different diameters (Supplementary Materials, Table S1). These results further confirmed the superior photocatalytic activity of the highly crystalline IO-CsPbBr 3 compared to its polycrystalline counterparts, and the beneficial effect of morphology on the photocatalytic performance was also observed. The photocatalytic activity of 2/IO-CsPbBr 3 may be attributed to two factors. Firstly, when the IO-CsPbBr 3 samples were prepared from polystyrene microspheres with different diameters, 2/IO-CsPbBr 3 had the smallest pore size and probably the most active sites. Secondly, as the void sizes and the wall thickness of 2/IO-CsPbBr 3 also decrease, the photogenerated carriers could be rapidly transferred onto the photocatalyst surface before their recombination in the bulk [19].
These observations were attributed to the higher crystalline quality of 2/AS-IO-CsP-bBr3, which contained lower defect density and fewer grain boundaries, and thus eliminated the recombination centers of the photogenerated electrons and holes [37]. Moreover, under the condition of adding the same weight of photocatalysts, 2/AS-IO-CsPbBr3 had fewer non-structured solids for its more regular inverse opal structures (Figure 2), which consequently increased the number of active sites [38]. Additionally, the polymerization of TerEDOT under illumination in absence of IO-CsPbBr3 powders is shown in Figure 6C, which reveals that TerEDOT undergoes extremely slow photolytic polymerization in the absence of a catalytic, causing a decrease in the TerEDOT concentration of approximately 4% within 45 min.
To further verify this phenomenon, photocatalytic polymerization catalyzed over other IO-CsPbBr3 (3/IO-CsPbBr3, 4/IO-CsPbBr3 and 5/IO-CsPbBr3) samples was studied. As shown in Figure 7A,B, all of the AS-IO-CsPbBr3 samples exhibit obviously higher photocatalytic performance than that of the corresponding RS-IO-CsPbBr3, which is consistent with the performance of 2/IO-CsPbBr3. Moreover, the photocatalytic performances of 3/RS-IO-CsPbBr3, 4/RS-IO-CsPbBr3 and 5/RS-IO-CsPbBr3 did not exhibit significant differences and were lower than that of 2/IO-CsPbBr3. The effect of morphology on catalytic performance was more significant in the case of AS-IO-CsPbBr3. In the presence of 3/AS-IO-CsPbBr3, 4/AS-IO-CsPbBr3 and 5/AS-IO-CsPbBr3, the absorbance at 452 nm decreased to 64%, 56% and 50% at 45 min, respectively. This observation applies, in particular, to the area of AS-IO-CsPbBr3 prepared using microspheres with different diameters (Supplementary Materials, Table S1). These results further confirmed the superior photocatalytic activity of the highly crystalline IO-CsPbBr3 compared to its polycrystalline counterparts, and the beneficial effect of morphology on the photocatalytic performance was also observed. The photocatalytic activity of 2/IO-CsPbBr3 may be attributed to two factors. Firstly, when the IO-CsPbBr3 samples were prepared from polystyrene microspheres with different diameters, 2/IO-CsPbBr3 had the smallest pore size and probably the most active sites. Secondly, as the void sizes and the wall thickness of 2/IO-CsPbBr3 also decrease, the photogenerated carriers could be rapidly transferred onto the photocatalyst surface before their recombination in the bulk [19]. In the infrared spectrum of the TerEDOT, the peak at 3104 cm −1 was attributed to the C=C−H stretching vibration, while it was not observed in the obtained crystal/polymer composite, which suggested the polymerization of the TerEDOT ( Figure S7). Templated structures are usually fragile and may undergo morphological changes or degradation In the infrared spectrum of the TerEDOT, the peak at 3104 cm −1 was attributed to the C=C−H stretching vibration, while it was not observed in the obtained crystal/polymer composite, which suggested the polymerization of the TerEDOT ( Figure S7). Templated structures are usually fragile and may undergo morphological changes or degradation under catalytic conditions. To evaluate the stability of the catalysts, 2/AS-IO-CsPbBr 3 was collected after reacting for 45 min. According to the SEM images, the spent 2/AS-IO-CsPbBr 3 samples appeared to be smaller, which can be explained by abrasion from stirring during the reaction. However, the open porous network and the inverse opal morphology were still well maintained, which confirms the morphological stability of the AS-IO-CsPbBr 3 ( Figure S8). Moreover, the XRD pattern of the spent 2/AS-IO-CsPbBr 3 is consistent with that of the freshly prepared 2/AS-IO-CsPbBr 3 , which also confirms the stability of the photocatalysts. In addition, the normalized absorbance UV/Vis spectrum and, therefore, also the band gap of the spent 2/AS-IO-CsPbBr 3 remained unchanged ( Figure S9). These observations demonstrate the excellent stability of IO-CsPbBr 3 in photocatalytic polymerization.

Synthesis of Colloidal Polystyrene Spheres (PSs)
Styrene (30 mL) was washed four times with 100 mL of 0.1 M NaOH in a separatory funnel and then four times with 100 mL deionized water. The standard synthetic procedure was as follows: 280 mL deionized water was poured into a 4-neck round-bottom flask and heated to 80 • C. Then 0.22 g of potassium persulfate was dissolved in 17 mL of deionized water and the obtained solution was injected after the prepared styrene was added. No cross-linking agent was added for the later removal of the latex spheres from the microporous product. The mechanical stirrer had a speed equal to 350 rpm with a variation estimated at 2 rpm, and the temperature was kept at 80 ± 2 • C for 12 h. After the polymerization was completed, the cooling colloidal solution was filtered through degreasing cotton to remove any large aggregations. Before use, the latex spheres were purified by repeated centrifugation/redispersion cycles and finally diluted to a mass fraction of about 8 wt%. In the above process, the spheres with diameters of 501 nm ± 20 nm, 433 nm ± 17 nm, 379 nm ± 15 nm and 214 nm ± 10 nm were produced with 45 mL, 30 mL, 20 mL and 8 mL of styrene, respectively. The diameters of the spheres were estimated using scanning electron microscopy (SEM).

Fabrication of 3D Ordered PS Template
The preparation of 3D ordered PS templates from monodisperse spheres with different diameters could be easily achieved by filtration. The colloidal dispersions were carefully poured onto a filter funnel preset with two suitable filter papers under vacuum. After 10-12 h, the face-centered cubic template was formed and dried at 50 • C overnight.

Preparation of Inverse Opal Structured Highly Crystalline Perovskite
A 0.45M precursor solution of the perovskite was prepared using equimolar amounts of CsBr (0.96 g) and PbBr 2 (1.65 g), dissolved in DMSO (10 mL) at room temperature. Then, a piece of face-centered cubic PS template (500 mg) was infiltrated by this solution (500 µL) and further treated with vacuum degassing for 10 min to make sure all the spaces between the 3D colloidal spheres were filled with precursor solution. The impregnated monolith was immediately moved into a 50 mL beaker with 10 mL methanol. This mixture was then treated with vacuum degassing for another 10 min before being transferred to a constant temperature incubator at 45 • C and atmospheric pressure for 24 h. After that, the color of the composition turned orange. The obtained compound was immersed in toluene (50 mL) at room temperature for 12 h, and the perovskite inverse opal was gathered by centrifugation. Tetrahydrofuran was also a good solvent due to its suitable solubility for the PS template. To ensure the PS template was completed removed, this washingcentrifugation process was repeated several times, and the obtained orange powder was dried at 50 • C overnight.

Preparation of Inverse Opal Structured Polycrystalline Perovskite
A 0.45 M precursor solution of the perovskite was prepared using equimolar amounts of CsBr (0.96 g) and PbBr 2 (1.65 g), dissolved in DMSO (10 mL) at room temperature. Then, a piece of face-centered cubic PS template (500 mg) was infiltrated by this solution (500 µL) and further treated with vacuum degassing for 10 min to make sure all the spaces between the 3D colloidal spheres were filled with precursor solution. The DMSO was removed under vacuum at 60 • C for 3 h. The obtained compound was immersed in toluene (50 mL) at room temperature for 12 h, and the perovskite inverse opal was gathered by centrifugation. To ensure the PS template was completed removed, this washing-centrifugation process was repeated several times, and the obtained orange powder was dried at 50 • C overnight.

Photocatalytic Polymerization of TerEDOT in Solution
The photocatalytic polymerization was carried out in a double jacket photoreactor with a quartz window for on-top illumination with a solar simulator (LCS-100). First, 20.0 mg TerEDOT and 10.0 mg Qu were dissolved in 20 mL dry toluene. Then, 60.0 mg IO-CsPbBr3 was added into the solution, and the mixture was illuminated for 45 min while the temperature was kept constant at 25 • C. A filter (20CGA-495, Newport) was used to cut off the wavelengths below 490 nm. To investigate the polymerization process, the reaction mixture was centrifuged at 3000 rpm for 2 min, and the supernate was diluted by 5-fold in toluene and used for UV/Vis spectrum measurement.

Characterization
Microstructures of the samples were characterized by field-emission scanning electronic microscopy (SEM, Hitachi FE-S4800, 5 kV), transmission electron microscopy (TEM, JEM-2000, 200 kV), high-resolution TEM images and selected area electron diffraction (SAED). Powder X-ray diffraction (XRD) spectra were recorded on a D8 advance X-ray Diffraction. The UV/Vis spectrum was recorded on a Lambda750 PE spectrometer. The infrared (IR) spectrum was measured on a BRUKER alpha II spectrometer. The highresolution mass spectroscopy (HRMS) was measured on BRUKER solanX 70 FT-MS, using ESI as an ion source. Proton nuclear magnetic resonance(1H NMR) was measured on a BRUKER AVANCE III 600M, using dichloromethane-d2 (99.5% atom% D) as solvent.

Conclusions
In summary, a series of highly crystalline CsPbBr 3 inverse opals were prepared by combining the colloidal crystal templating method and anti-solvent crystallization. The obtained IO-CsPbBr 3 exhibited a well-connected porous structure and high crystallinity. As a result of these advantages, a significant enhancement of photocatalytic performance was observed when the highly crystalline IO-CsPbBr 3 was used as a photocatalyst to promote the polymerization of TerEDOT. Moreover, IO-CsPbBr 3 is stable under photocatalytic conditions in terms of both morphology and crystal structure. The results indicated that the novel highly crystalline IO-CsPbBr 3 could be an attractive material in the field of heterogeneous photocatalysis.  Figure S8: The SEM image of the spent 2/AS-IO-CsPbBr 3 in low magnification. Figure S9: XRD patterns (A) and Normalised absorbance UV/Vis spectra (B) of the spent 2/AS-IO-CsPbBr 3 .